As shown in FIG. 20, a typical magnetic disk unit 71 that records data on a disk includes, in a housing 75 of a metal such as aluminum alloy, a head assembly 76 consisting of a slider 72 and a suspension 78, a disk medium (hereinafter referred to a disk) 73 as a recording medium, a disk rotation driving section 74 such as a spindle motor, and an actuator mechanism 77 which moves the head assembly 76 across the information recording surface of the disk 73. The slider 72 has a recording/reproducing element which records and reproduces data on the disk 73 and is flown above the disk 73 by airflow generated by rotation of the disk 73 at a minute height. The slider 72 has a positive-pressure generating section and a negative-pressure generating section on the surface facing the disk 73 and designed so as to fly with a constant flying profile over the entire area of the disk 73 under a constant temperature and pressure by a static load from the suspension 78 and the balance between the positive and negative pressures generated from the slider 72.
In order to increase the recording density of a magnetic disk unit, it is important to reduce the flying height, which is defined by the gap between the slider with the recording/reproducing element and the disk. Presently, the flying height is as small as 10 to 15 nm and there is a demand for reducing the possibility of contact between the slider and disk by minimizing variations in the flying height caused by environmental changes, in view of the reliability of the magnetic disk unit. Furthermore, magnetic disk units are used in growing numbers of portable devices such as notebook personal computers. For magnetic disk units for portable devices, it is demanded that decreases of the flying height caused by environmental changes, especially air pressure changes in highlands, be reduced. As the flying height decreases significantly, the possibility of contact between the slider and disk increases. Especially, when the flying height decreases below a glide height, which is determined by the surface roughness of the disk, the possibility of contact between the slider and disk radically increases. At worst, the information recorded on the disk can be corrupted by a slider crash.
There is a conventional technique for minimizing decrease of the flying height caused by an air pressure variation in places such as highlands, in which a magnetic disk unit is sealed in a glass container filled with helium gas at a pressure lower than atmospheric pressure, a pressure variation is measured with a pressure sensor in the glass container, and the pressure inside the container is controlled by heating or cooling so as to be kept constant, thereby reducing a variation in flying height caused by an air pressure change in the external environment (for example Japanese Patent Laid-Open No. 6-176557).
A technique concerning slider structure is disclosed in Japanese Patent Laid-Open No. 11-149733, for example, in which the central cavity and the side cavities on the air bearing surface of a slider are different in depth so that negative pressure is kept in balance, thereby reducing a variation in flying height caused by an air pressure change in the external environment.
Another technique is disclosed in Japanese Patent Laid-Open No. 2000-57724, No. 2001-283548, and No. 2001-283549, in which a proper ratio of the depth of a submicron-deep front step bearing to the depth of a negative-pressure cavity is chosen to reduce a variation in flying height caused by an air pressure variation in the external environment.
Yet another technique is disclosed in FIGS. 1 and 7 of Japanese Patent Laid-Open No. 2001-297421, for example, in which a magnetic head slider has four planes at different depths, and the depth of the negative-pressure cavity is chosen to be in the range of approximately 400 nm to 1.3 μm, thereby reducing the difference in flying height between a lowland and a highland (at an altitude of 3,000 m) to approximately 2 nm (20% of the flying height).